A cyclic olefin copolymer has excellent thermal resistance, mechanical characteristics, transparency, dielectric characteristics, solvent resistance, moldability, dimensional stability, and the like, and is used in various fields. However, depending on use, sometimes a higher degree of thermal resistance, solvent resistance, or mechanical strength is required. An attempt at further improving thermal resistance, solvent resistance, and mechanical strength by crosslinking the cyclic olefin copolymer by various methods such as crosslinking using sulfur, crosslinking using an organic peroxide, crosslinking using electron beams, and crosslinking using radiation has been made (Patent Documents 1 and 2).
However, recently, in the field of molding materials represented by electronic parts, a higher degree of thermal resistance has been required. In order to satisfy the requirement, a method of adding a large amount of inorganic substances (Patent Document 3), or a method of performing addition polymerization using only cyclic olefin monomers (Patent Document 4) has been proposed. However, generally, inorganic fillers have a problem that they greatly decrease toughness, dielectric characteristics, and transparency of organic materials. In addition, since a polymer obtained by performing addition polymerization by using only cyclic olefins has a glass transition temperature (Tg) that is excessively high, it is practically impossible to mold the polymer by melt processing, and molding the polymer into an arbitrary shape is industrially difficult. Moreover, when the polymer is used as a substrate for a liquid crystal display or a solar cell, vapor barrier properties or processability thereof needs to be improved.
Patent Document 5 discloses an α-olefin.cyclic olefin.polyene copolymer. However, it does not disclose a crosslinked polymer in detail, and still leaves scope for improvement in respect of improving thermal resistance, vapor barrier properties, and processability while maintaining excellent dielectric characteristics, transparency, and moldability.
Patent Document 6 discloses an α-olefin.non-conjugated cyclic polyene copolymer and a crosslinked polymer thereof, but the content of a constituent unit derived from the non-conjugated cyclic polyene is as small as 0.01% by weight to 20% by weight. Patent Document 7 discloses a crosslinked polymer of a curable resin that contains C2-20 α-olefin and a cyclic olefin having two or more C═C double bonds as constituent units, but does not discloses the content of the cyclic olefin having two or more C═C double bonds. Moreover, although the present inventors tried to polymerize the curable resin according to the method described in Patent Document 7, the thermosetting resin could not be obtained. Therefore, regarding the above, the crosslinked polymer disclosed in Patent Documents 6 and 7 still leaves scope for improvement.
In addition, Patent Document 8 discloses a cyclic olefin copolymer having a crosslinkable group that contains a repeating unit derived from olefin and a repeating unit derived from a cyclic non-conjugated diene in a predetermined molar ratio and a crosslinked polymer thereof. However, dielectric characteristics of the crosslinked polymer obtained from the cyclic olefin copolymer disclosed in Patent Document 8 sometimes deteriorate over time.